Ref: CPO Inquiry Document Inq /13b, dated 18th January 2024: Note-to_Inspector:
Section 9:
9 AP: TWIN PIPE
9.1 This alternative proposes installing two parallel 2m diameter pipes, 4m below existing ground levels which would run from a new pumping station north of Botley Road to a discharge point near the A423 Oxford Southern Bypass.
HOEG Note: in Inquiry Document: CD2.12-VBA-review-of-HOEG-twin-pipe-proposal-29.06-1
EA preferred contractor, VBA, describe a 32m-wide ‘open cut’ trench for the pipes:
Area calculation:
- i) VBA Design: 32m-wide trench at ground level:'
- ii) HOEG Design: 8m-wide trench at ground level:
Current scheme design (1:3 gradient)
Drwg ref: ‘typical cross section of channel at Area 3A’ (P01). Drwg allows for 1 in 3 gradient
- Area calculation
- proper opening width: (depth x 2) x type slope ratio + width of original excavation = top width
- ((4m x 2) x 3)) + 8m wide = 32m wide excavation at top
- Based on 8m base + 32m top - Area of trapezoid = 80m2
- Total approx. 80m2 x 5km = 400,000m3 (VBA/EA)
- HOEG Total: 150,000m3, of which ca. 35,000m3 remains on site as a shallow (0.5m trapezoidal) bund over the backfilled trench.
HOEG Comment: VBA/EA’s proposal is an extremely inefficient method of trenching for pipe-laying, resulting in a ca. 10x over-excavation of material. (350,000m3 vs. 35,000m3.) This will also result in considerably more upcast (ca. 90,000m3 vs. 0m3) being removed from site.
Para. by Para. refutation of VBA/EA’s proposal, Section 9. Of Document Inq /13b, Note-to-Inspector-on-Alternatives-18-January-2024, follows:
9.1: HOEG Reply: The depth of the twin parallel pipes is 2m to top of pipes, below ground level; 4m to base of pipe trench, dipping to 5m below stream crossings.
Environment Agency
9.2 This alternative is addressed in detail in the Proof of Evidence of Richard Harding (EA/1a) at paragraph 15.11.
9.3 This alternative is also addressed in detail in the Rebuttal Proof of Evidence of Phil Raynor at paragraph 6 (EA/5d) and the Additional Rebuttal Proof of Evidence of Phil Raynor (EA/5e) at paragraph 6.
9.4 In relation to the merits and practicability of the alternative:
In Richard Harding’s evidence:
Environment Agency
(a) EA guidance promotes, as far as possible, low risk passive designs instead of active ones, as per Operational Instruction OI 1418-12 (EA-RNH25, which is at EA/1c.3).
(b) This alternative is effectively a form of culverting. The EA’s policy position is not to culvert watercourses, as per Passive Design Guidance Operational Instruction OI 1418-12 and Culverting Watercourses 170_19 (EA-RNH26, which is at EA/1c.3).
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(c) This alternative is considered prohibitively expensive, at an estimated cost of £86m - £106m (as described in the VBA review of this alternative at CD2.12). This alternative would also result in a high lifetime maintenance and carbon cost.
9.4 (c): HOEG Reply: EA’s costing of the Twin Pipe pipe alternative is too high because they failed to put it out to multiple, independent tender. More than one contractor may be considered for the work: e.g. i) trenching; ii) pipe-laying; iii) pump-house construction; iv) pump and manifold installation.
An estimate by single, preferred contractor, VBA, of £86m – £106m is much too high. This type of engineering work is largely unfamiliar to EA. Cost estimates by several specialist firms for trench construction and backfill; pipe-laying; pump-house construction; and pump and manifold installation would lead to a greatly reduced estimate of total cost.
As an example, an estimate from Future Pipes for delivery to site of 5km (x2) of 2m nominal diameter sleeved GRP pipes, was for less than €2m.
As a comparison, the Thames Water London Ring Main, which is an underground conduit 10m to 65m deep, and 2.5m in diameter, only cost £11m/mile (2021 prices), and is far more complex than alternative AP.
Kevin Larkin and I, on behalf of HOEG, think that our estimate of £22m (2022) is realistic.
Annual maintenance is expected to be straightforward and relatively inexpensive. Viz.: checking of: pipe integrity; backup electrical power; and correct operation of pumps and manifold valves.
Environment Agency:
(d) Additional uncalculated costs arise from the need to cross the railway, and construct a viaduct, in order to get water to the discharge point.
9.4 (d): HOEG Reply: We do not specify crossing the railway in alternative AP. The discharge point is specified as being immediately beyond Old Abingdon Road. The cost of a new railway viaduct, and changing the discharge point to E of the railway, has not been costed in alternative AP.
Environment Agency:
(e) From a technical perspective, the velocity of flow would be significantly faster at over 7m/s3. This would lead to downstream impacts without significant infrastructure, such as a stilling basin, to reduce velocities. Without such a structure, there would be impacts on the watercourse and its bed and banks downstream from scour created by the high velocities. It is also likely that further attenuation of flows would still be required to prevent an increase in flood risk to communities further downstream (at Abingdon) with no clear mechanism as to how to achieve this.
9.4 (e): HOEG Reply: We have designed, but not submitted to the Inquiry, a discharge race that will reduce flow velocity to less than 2m/second. This will disperse kinetic energy by means of baffles set into concrete, and is a standard method of slowing fast-moving water. A stilling pond will not be required.
Attenuation of flow in order to protect communities further downstream at Abingdon would have the same priority as for OFAS. Alternative AP has, furthermore, a better ability to control flow rate than has OF
Environment Agency
(f) The proposed inlet sump and pumping station would also be in the Green Belt and floodplain. The pipeline route would also cross through the centre of Hinksey Meadow’s MG4 grassland. The excavation works and construction corridor required to deliver this alternative would destroy much of the grassland.
9.4 (f): HOEG Reply: The proposed site for the pump-house in in a field adjacent to Seacourt Park and Ride. The field is of lower ecological sensitivity than Hinksey Meadows, and the pump-house would occupy a comparatively small area within it, of ca. 1,500sq.m.
The route for the Twin Pipes can be adjusted to minimise encroachment onto sensitive grassland. The route would pass to the East side of the Meadows, parallel to the Electric Road. Minimal destruction of MG4 grassland would be necessary, with trench back-fill and re-instatement leaving little trace of construction.
Environment Agency:
(g) Debris management at the pumping station intake structure would lead to accessibility issues during a flood event.
9.4 (g): HOEG Reply: Debris management during a flood event can be accommodated into the flow inlet design. Debris is ‘flotsam’, i.e. floating material, and will not obstruct underwater flow through the inlet grilles. The curved shape of the inlet spillway will tend to disperse debris to downstream, and the area to N of the pump house, between the A34 and Binsey Lane, is relatively free of trees and shrubs.
Environment Agency
(h) The alternative proposed does not address the extensive groundwater issues that would occur from excavating a 4.0m deep trench into the floodplain.
9.4 (h): HOEG Reply: The alternative AP proposal presented to the Inquiry indirectly addresses ground water movement across the line of the pipeline.
Transverse ground water movement will not be obstructed by pipeline emplacement. Backfill of the pipeline trench will include excavated, porous and permeable, Thames Gravel. This will underlie the pipes and occupy the lower part of the backfilled trench, preserving existing stratigraphy, in order that ground water levels equilibrate as before the trench was made, and maintain the ability of ground water to flow across the line of the buried pipes.
Environment Agency
(i) The VBA assessment at CD2.12 identifies a cross-section for works requiring a 32m wide working strip, plus working areas either side, contrary to HOEG’s assertion that this would require a strict 8m width for works.
9.4 (i): HOEG Reply: Reference to standard pipeline trenching methods shows that a narrow strip of land of about 4m width is required on each side of the trench for vehicular movements. This gives a maximum of 8m width of trench, and 4m + 4m strips to either side, which will be reinstated on completion of work.
EA-preferred contractor, VBA, may have limited experience of laying a pipeline for a short distance across flat meadowland. Pipelines often extend for tens or hundreds of miles, across uneven and inhomogeneous substrate. The techniques of pipe-laying are well established, and have been refined to minimise costs.
Environment Agency:
(j) It is not clear from the information provided how the inlet or discharge points would actually work.
9.4 (j): HOEG Reply: We have drawn up a design, based on standard engineering practice, for a discharge race that will disperse water flow kinetic energy by means of baffles.
Environment Agency; In Phil Raynor’s evidence:
(k) The trench would impact on groundwater flows and levels within Hinksey Meadow, which could either create a preferential flow path or block groundwater flow paths. Either outcome would change the hydrogeological regime and potentially affect the grassland communities and biodiversity.
9.4 (k): HOEG Reply: The comment about ground water flow is addressed in 9.4 (h) above.
Environment Agency
(l) A preferential flow path at a depth greater than the CPO Scheme could have a potentially larger impact by reducing groundwater levels over a wider area.
9.4 (l): HOEG Reply: On completion of pipe-laying works, and backfill of the trench, ground water levels will restore to the same as before the trench was made. The will be no lowering of ground water levels over the proximal and wider areas.
Environment Agency:
(m) Limited information is provided about how the in-take structure would work hydraulically. The majority of the flow within Seacourt would need to pass towards the oval in-take weir, which would have consequential impacts on the watercourse.
9.4 (m): HOEG Reply: EA/Phil Raynor’s comment needs to be clarified. There is no predicted adverse effect of the Seacourt pump house water inlet on the watercourse. Water level along the watercourse will be kept to a lower level during a flood than would be the case with OFAS.
Environment Agency:
(n) The discharge location may impact on flood risk elsewhere, and may not be an appropriate location for such a structure or for any residual risk of erosion.
9.4 (n): HOEG Reply: The effects of the discharge race have been addressed in 9.4 (e) above.
Environment Agency
(o) Wider implications on the environment, including laying the pipelines across Hinksey Meadow particularly, have not been factored into this alternative.
9.4 (o): HOEG reply: The wider implications for laying a pipeline across the Meadows, towards to East side and parallel to the Electric Road, are considerably less than for the wide OFAS channel across Hinksey Meadows to the West. The pipeline trench can be backfilled and reinstated, leaving little evidence of the underlying Twin Pipes.
Environment Agency
(p) For these reasons, this alternative is considered to have limited merit and is impractical.
9.4 (p): HOEG Reply: Alternative AP has the considerable advantage over OFAS in that it is far less destructive of MG4 and other sensitive grassland; requires much less land area to construct, and which can be re-instated to near-original ground appearance; is less expensive and quicker to construct; requires few temporary land requisitions, and a CPO only over the land required for the pump house and discharge race.
9.5. In relation to whether Alternative AP would meet the aims and objectives of the CPO scheme:
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Environment Agency
(a) The EA does not have sufficient information to conclude that this alternative would meet the aims and objectives of the CPO Scheme.
9.5 (a): HOEG Reply: The EA has not requested from HOEG any information about the Twin Pipe alternative, AP. EA has access to HOEG design drawings, engineering volumetric estimates, and costings.
Environment Agency
(b) In any event, for the above reasons, the alternative would not be a viable or practicable long term method of meeting those aims and objectives.
9.5 (b): HOEG Reply: Alternative AP meets the aims and objectives for medium to long term flood defence for West Oxford, in a cost-effective manner. It can be constructed over a shorter timescale than OFAS, while causing minimal damage to the sensitive grassland environment.
It may, in principle, be removed/reversed at some later date, leaving the landscape in a more or less identical state as before construction, unlike OFAS which leads to permanent and irrecoverable damage to Hinksey Meadows.
Alternative AP has much more flexibility than OFAS in terms of detailed design. For example:
- Maximum flow rate can in principle be increased, by use of larger pipes.
- The Twin Pipe route can be positioned so as to optimise the balance between expediency of construction and impact on the Meadows area.
- Water flow rate during floods can be finely tuned, with pumping during the early days of a flood reducing peak flood water levels.
- The discharge point for one or both pipes can be extended to a distance downstream, in principle as far as the downstream side of Sandford Weir.
Environment Agency
(c) This alternative would not avoid the need for compulsory purchase.
9.5 (c): HOEG Reply: Alternative AP removes the necessity for the great majority of Compulsory Purchase Orders over the Hinksey Meadows area.
Temporary requisition of land for trenching is only necessary during pipe-laying, and then over a relatively small area.
There will be a need for Compulsory Purchase, however, for the pump-house site adjacent to Seacourt Car Park, to W.
Jonathan Madden
HOEG
1st February 2024